1. Field of the Invention
The present invention relates to a dielectric constant sensing device for sensing a dielectric constant of a fluid to discriminate the property of the fluid and a method employing the same and, more particularly, a device for measuring an alcohol containing rate of an alcohol blended fuel employed in an engine of an automobile, etc. and a method employing the same.
2. Description of the Prior Art
In recent years, in order to promote the employment of the alternative fuel, automobiles which can use the fuel which is prepared by blending alcohol such as methanol, ethanol, etc. into a gasoline are being introduced. If an engine can employ the alcohol blended fuel is controlled, the air-fuel ratio of the engine, the ignition timing, or the like must be changed according to the alcohol concentration in the fuel in order to clean an exhaust gas or to extract a sufficient engine power, unlike the case where an engine can only employ the gasoline is controlled. Since the dielectric constant of the alcohol blended fuel is changed according to the alcohol concentration, the alcohol concentration can be detected by measuring the dielectric constant. For this purpose, in the prior art, for example, the dielectric constant sensing device disclosed in U.S. Pat. No. 5,255,656 has been proposed.
This device will be explained with reference to FIGS. 8, 9, and 10 hereinbelow. FIG. 8 is a plan view, partially broken away, showing a dielectric constant sensing device in the prior art. FIG. 9 is a cross sectional view showing the dielectric constant sensing device in the prior art. FIG. 10 is a block diagram showing a circuit configuration of the dielectric constant sensing device in the prior art.
In FIG. 8, a reference 301 denotes an input pipe into which the fuel is introduced; 302, an output pipe for discharging the fuel; 303, a sensor portion; and 304, a sensor circuit chamber (omitted partially) in which a sensor circuit 400 for processing signals from the sensor portion 303 is built. In FIG. 9, a reference 305 denotes an outer wall of the sensor portion 303 formed of resin, and an electrode 306 which is formed of double layers of copper and nickel by the plating is provided on an entire inner surface of the outer wall 305. A reference 307 denotes a chamber which is filled with the fuel, and a reference 308 denotes a cylindrical coil which is provided in a coaxial manner relative to an inner surface of the outer wall 305 and has the inductance L0. This cylindrical coil operates as an electrode, and has a capacitor of capacitance C0 which is a sum of a stray capacitance formed between the coil wires and a capacitance formed between the electrode 308 and the electrode 306.
In FIG. 10, the electrode 306 is connected electrically to ground of the sensor circuit 400. Both ends of the cylindrical coil 308 are connected to the resonator circuit 401 which is composed of a CMOS inverter. An output of the resonator circuit 401 is connected to the output circuit 402. A reference 403 denotes a power supply circuit 403 for supplying a constant stabilized voltage to the overall sensor circuit, and a reference 404 denotes a temperature measuring circuit which has a thermistor used to execute the temperature compensation.
Next, an operation will be explained. The inductance L0 of the cylindrical coil 308 and the stray capacitance C0 constitute a parallel resonance circuit which can resonate at a resonance frequency Fr given by ##EQU1##
Since the resonance circuit 401 is formed to feed back positively at the resonance frequency Fr, oscillation at the resonance frequency is continued. This resonance frequency is divided by the output circuit 402 and then transmitted to an engine control device (not shown).
If the fuel is filled in the chamber 7, the stray capacitance C0 is changed by the dielectric constant and thus the resonance frequency is changed according to Eq. (1). Therefore, the dielectric constant of the fuel can be detected by sensing the resonance frequency. Since the dielectric constants of the gasoline and the methanol are given as about 2 and about 32 respectively in the methanol blended fuel, change in the methanol concentration contributes significantly to change in the dielectric constant of the fuel. Hence, if the dielectric constant of the fuel can be detected, an alcohol blended ratio can be detected, so that appropriate engine control can be achieved.
However, since the dielectric constant sensing device in the prior art is constructed in the following, there have been problems described later.
Since size of the sensor 3 is not increased so much with regard to the mountability as the engine control device for the automobile, the magnitude of the stray capacitance being formed is limited by itself. In the prior art set forth above, the stray capacitance C0 of the cylindrical coil 308 is only 26 pF. In contrast, the input portion has the stray capacitance having the unnegligible magnitude. For example, if the voltage at the terminal of the cylindrical coil 308 is input into any IC, the input capacitance of several pF exists for every IC. In addition, if the wirings are provided on the circuit substrate, there is the case where the capacitance of several pF is generated only by tne wirings. The stray capacitance at the input portion of the sensor circuit 400 is input in parallel with the stray capacitance C0 of the cylindrical coil 308 in circuit, the stray capacitance C0 is changed correspondingly to thus change the resonance frequency Fr of the sensor.
In addition, such stray capacitance is not a controlled and stabilized capacitance and thus varied easily by the peripheral temperature and the deterioration of the durability. Therefore, there has been the problem that such stray capacitance becomes a factor to cause the error in the sensor output.
In order to employ the cylindrical coil 308 as the inductor as a circuit element, i.e., as a circuit element for generating a voltage, which is proportional to the time differential of the current flowing through the element, between both terminals of the element, both terminals of the cylindrical coil 308 must be connected to the sensor circuit 400. Therefore, there has been the problem that the number of the terminals is increased.
In addition, there has been the problem that, since several electronic parts are needed at the minimum in order to constitute the resonator circuit, a circuit scale is enlarged.